Under the Prenatal Sun

Pregnant women who get a reasonable amount of sun and so have good vitamin D levels during the third trimester have children with stronger, bones.

Pregnant women can help their babies build strong bones even before birth, according to a recent study. These prenatal influences on fetal bone development can still be seen at 9 years of age and may well persist for many more decades. Third trimester maternal vitamin D levels, modulated through sun exposure, may have a "programming" effect on the developing fetal bone growth resulting in a life−long increase in the content and density of minerals in the bones. If this is, as suspected, persistent throughout life, these stronger bones may decrease the risk of osteoporosis when the infants become grandparents.

The primary function of vitamin D is to maintain normal blood levels of calcium and phosphorus, and to promote bone mineralization.

There have been numerous studies linking prenatal influences and bone health in later life. Many of them have found a link between sun exposure (whether through latitude or season) and bone growth and strength.

The primary function of vitamin D is to maintain normal blood levels of calcium and phosphorus, and to promote bone mineralization. Vitamin D, a fat−soluble vitamin, is produced by the body when exposure to ultraviolet B (UVB) radiation from the sun converts a chemical in the skin, 7−dehydrocholesterol, to previtamin D3, which then becomes vitamin D3. The recommendations for the daily allowance of vitamin D have been increased reflecting the appreciation of the importance of this vitamin and the realization that in many latitudes and during many seasons, sun exposure is not sufficient to provide adequate levels. Food sources of vitamin D are few and include fatty fish such as salmon, mackerel and sardines, cod liver oil, and fortified milk.

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Bone growth begins in the eighth week of embryonic development. Most bones are made first of cartilage which is gradually replaced by bone tissue. After birth, bone length is increased by the production of cartilage which is replaced by mineralized bone, a process that continues most actively through puberty. The width of bones is increased when the periosteum, the connective tissue membrane that covers the bone, secretes more bone tissue that is added to the surface of the existing bone.

Bone functions to support the body and to store minerals that are needed for the body's biochemical functions. The flexibility of bones is provided by protein and collagen, and the mineral content is primarily of calcium and phosphorus. Resorption and remodeling of bone continue through life, although resorption outpaces remodeling in the later years, contributing to loss of bone mass and strength. It is the process of bone growth and remodeling, and the periosteal role in increasing bone mass and width that the researchers are hypothesizing is "programmed" during third trimester by exposure to sunlight and the influence of vitamin D levels.

Approximately 7000 pregnant women who were due between April 1991 and December 1992 were recruited into the study. They were an ethnically homogeneous, Caucasian population in England. Their babies' weight and head to heel length were measured within the first 3 days of life. When they reached 9.9 years, they were evaluated with total body DXA scans which measure bone mineral content, bone area, and bone mineral density. Their total body fat and lean mass (the weight of bones, muscles and organs or, everything but the fat), and their heights and weights were also measured. Maternal sunlight exposure during the third trimester (98 days before birth) was measured with meteorological records and a statistical model was used to relate sunshine to UVB exposure. Mothers' blood levels of vitamin D were measured at 36.6 weeks gestation and strongly paralleled the meteorologically estimated UVB exposure.

Their results showed that UVB exposure during the final trimester was strongly associated with longer length but not increased weight, at birth. At 9.9 years, UVB exposure was strongly associated with increased height, weight, and lean body mass. The bones were longer or larger, and heavily mineralized with calcium and phosphorus when measured at 9.9 years. Since their study population was quite large, they were able to statistically separate the effect of UVB light on height from its effect on bone mineral density itself. They showed that the effect on bone mineral content and bone area were primarily related to bone size and that 50% of the effect of UVB on bone size was in width and mass rather than height.

The significance of this study, published in the March issue of the Journal of Clinical Endocrinology and Metabolism, suggested the exciting possibility that this prenatal exposure's effects persist throughout life. Since previous data have shown that the risk of fracture clearly correlates with decreases in bone mass, the researchers suggest that if their findings at 9.9 years of age persist into adulthood, it is likely that as maternal UVB exposure increases, later fracture risk decreases.

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Sun exposure has been a controversial issue as more and more studies highlight positive effects of Vitamin D and the potentially harmful and carcinogenic effects of prolonged sunlight exposure. Sunbathing to increase skin vitamin D production is recommended, but researchers are interested in how much sunlight is needed to produce adequate levels. During the winters, in some latitudes, it is not possible to get enough sun exposure to meet the needs for vitamin D. Some researchers have suggest that 5 to 30 minutes of sun exposure between 10am and 3pm twice a week is sufficient, but latitude, season, skin covering, sunscreen and other factors influence this.

It makes sense to be aware of one's dietary intake of Vitamin D, to attempt to eat vitamin D rich foods and enriched products, and to check with one's physician about the age− specific recommendations for supplementation. It is also possible to measure blood vitamin D to help in monitoring nutritional adequacy. This study strongly supports paying much closer attention to vitamin D levels in pregnant women as this nutrient may have a positive effect in producing stronger and better mineralized bones that will last a lifetime.